The present invention relates to vacuum-degasing of liquid metals in particular of liquid steel and, more specifically, to degasing by vacuum jet.
There are known several methods of vacuum degasing, said methods being recalled in the article by M. VERGE entitled "Tendances actuelles des procedes de degazage d'acier liquide" (Heurtey, Bulletin d'information n.degree. 42 of March 1968) and, in particular, the method of vacuum-jet degasing according to which (FIGS. 1 and 2) the liquid steel to be degased 1, contained in a ladle 2, flows (arrows 1a) through a port 2a provided at the bottom thereof and is admitted through a nozzle 3 into an enclosure 4 held under vacuum (with the air being evacuated in the direction shown by arrow 5 through a port 6). The liquid jet 7 admitted into the evacuated enclosure 4 bursts to droplets 8, said droplets being collected either in a ladle, or in an ingot mold 9 in which degased steel 10 is collected.
The vacuum degasing in particular by vacuum jet of steel is used in order to improve the quality of finally obtained steel. Such a process chiefly aims to substantially reduce the hydrogen content in the liquid metal, thus preventing flaking and it is known that the presence of flakes reduces the mechanical properties, especially the life in fatigue tests, as well as the oxygen and nitrogen contents, thus reducing segregations and inclusions (the vacuum deoxidizing enabling the number of metallic quenching additions to be reduced), hence improvement of mechanical characteristics in longitudinal direction and especially in transverse direction. For certain steels, including stainless steels, the vacuum degasing allows a reduction of other gas content, e.g. of carbides present therein.
However, the known methods of vacuum casting have certain drawbacks. The metal jet escaping through port 3a provided in the nozzle 3 is abruptly brought to a very low pressure (which may be in the range of 1 torr, i.e. 1 mm of mercury) prevailing in the enclosure 4. There results violent boiling of liquid metals with very quick forming and growing of gas bubbles and a bursting of jet 7 which is dissociated into fine droplets having a diameter in the range of a fraction of millimeter. While this dispersion of the jet into small droplets provides a considerable exchange area between steel and air and accordingly an excellent and almost instantaneous degasing, it has however two drawbacks which limit the development of the technique of degasing by vacuum jet, viz:
on the one hand, a substantial cooling of the metal, due to the large exchange area between the metal and the thus-created rarefied atmosphere; and
on the other hand, metal projections onto the walls of chamber 4 and, in the case of casting in an ingot mold, onto the walls of the latter; these projections onto the walls are cooled quicker than the bulk of the metal falling in the center of the ingot mold, and, since they have been subjected to a different thermal treatment, they originate in defects in the final ingot cast in the mold.